Various control schemes exist for driving electrical current through an associated load. The particular scheme or combination of schemes generally depends on the type of load and performance requirements of the system. For example, two known approaches for driving a load include Pulse Width Modulation (PWM) and linear current control. In linear control, the current is driven as a linear function of an input signal, such as a control voltage. For PWM control, the load is driven between alternating high and low voltages to control electrical current in that load. Usually, a PWM scheme utilizes power switch devices (e.g., field effect transistors) coupled across the load. The PWM control system selectively activates the switch devices from high to low conditions or from low to high conditions to electrically couple the load to an associated voltage level. Various power applications can employ a PWM scheme, a linear scheme or a dual PWM and linear scheme to provide desired current and/or voltage to an associated load.
By way of example, magnetic disk drives remain a dominant technology for mass read/write storage in modern computers, including both desktop computer workstations, portable computer devices (e.g., laptop computers, portable audio systems and players), and various types of set top boxes for a variety of applications (e.g., gaming as well as satellite and cable television). A hard disk drive generally includes one or more rotating disks or platters and a spindle motor that is controlled to cause the disks to rotate. Data is generally stored in the form of a sequence of magnetically polarized regions on the surface of the disk. One or more read/write heads, which is sensitive to changes in magnetic flux, reads and/or writes data to the disks as it is supported by an arm above the surface of the disks in close proximity relative to the disks. An actuator motor (typically a voice coil motor (VCM)) controls the positioning of the arm for moving read/write heads relative to the surface of the disks.
Voice coil motor control circuitry generally provides drive signals to the voice coil motor through one or more stages of driver circuitry. The driver circuitry is coupled to drive switch devices connected on opposite sides of the voice coil motor. In operation, the positioning arm is moved radially along the surface of the disk in one direction by driving current through the voice coil motor in one direction, and moved radially in the opposite direction by the driving current through the voice coil motor in the opposite direction. The amount of current through the motor, regardless of current direction, generally determines the speed at which the positioning arm is moved. Generally, a linear control scheme is employed for current near a zero-crossing point (e.g., slow speeds) and a PWM control scheme is employed when greater amounts of current are required (e.g., for higher speeds).